Sanitary sewer systems are generally designed to carry away wastewater from sinks, dishwashers, showers, and toilets from homes or businesses through a system of buried pipes, often referred to as “sewer lines”, to a sewage treatment facility. Such sewer systems are generally separate from storm drains and catch basins used for collecting rain water or “clear” water runoff. Infiltration and inflow (“I/I”) of rain water or clear water, terms used to distinguish it from wastewater, (although clear water may be dirty), to sewer systems increases the load on sanitary sewer systems. Clear water typically is collected and directed via storm drains, culverts, drainage ditches, or across ground surfaces, but generally not in sanitary sewers. When clear water enters sanitary sewer systems, it must be transported and treated like sanitary waste water which increases the load and thus the cost associated with sanitary sewage systems and treatment. During dry weather, the amount and impact of I/I varies based on the circumstances. During wet weather, the impact of I/I is typically increased as the precipitation makes its way into the ground and often into sewage systems. As a rain or snow melt event occurs, I/I sources can start filling the sanitary sewer systems with clear water, and can eventually fill the sewer systems to capacity.
Inflow and infiltration are generally understood to mean two different things. Inflow is a direct connection or communication of clear water to a sewer system. Various sources contribute to inflow, including footing/foundation drains, roof drains or leaders, downspouts, drains from window wells, outdoor basement stairwells, drains from driveways, groundwater/basement sump pumps, and even streams. These sources are typically improperly connected to sanitary sewer systems, either directly or indirectly through wastewater drains. An improper connection allows water from sources other than the appropriate wastewater sources to enter the sanitary sewer system. This clear water should be entering storm water systems or be allowed to soak into the ground, instead of entering the sanitary sewer system.
Infiltration on the other hand occurs through cracks and/or leaks in the sanitary sewer pipes or manholes. Cracks or leaks may be caused by age related deterioration, loose joints, poor design, installation or maintenance errors, damage, or root infiltration. Groundwater enters these cracks or leaks whenever the soil above or surrounding sewer systems becomes saturated due to precipitation or nearby bodies of water. Often sewer pipes are installed beneath creeks or streams because they are the lowest point in the area and it is more expensive to install the pipe systems beneath a roadway. These sewer pipes are especially susceptible to infiltration when they crack or break and have been known to drain entire streams into sanitary sewer systems.
There are significant concerns that go hand-in-hand with I/I. Added clear water within sewer systems reduces the ability of those systems to operate properly. It increases the overall amount of wastewater in pipes as it combines with the waste that has properly drained into the sewers. This in turn increases the amount of wastewater that ends up at treatment facilities, thereby affecting the facilities' ability to properly transport, treat, and manage the end product. As a result of I/I, wastewater treatment processes are compounded and poorly treated wastewater may be discharged to the environment.
Problems can also occur before the wastewater arrives at the treatment facility. If sanitary sewer systems reach capacity or become overloaded, wastewater may flow backward through the sanitary sewer pipe, which can cause floods of basements or households and also cause manholes to pop open releasing wastewater onto the street. Overflow occurrences put public health at risk and violate state and federal environmental regulations. Overflows can release wastewater into waterways, onto streets, and even into basements, resulting in serious health risks. As wastewater overflows into creeks, rivers, lakes, and streams it contaminates all bodies of water downstream and affects all of the creatures and plants that come in contact with the polluted water.
There are various costs resulting from the effects of I/I, including the greater load to the sewer system pipes, sewer system overflows, and extra wastewater at treatment facilities. Accordingly, municipalities and sanitary sewer authorities have a financial interest in reducing I/I through repair or replacement of the sewage pipes. However, wastewater infrastructure maintenance is very costly, which has increased the interest in devices and methods for measuring I/I related flow in order to pinpoint the problematic sources.
There are multiple devices and methods that measure flow within sewer systems. Many of the available methods make use of a primary flow restricting device in conjunction with a sensing probe (generally an area-velocity meter or ultrasonic level detector). Examples of primary devices include the Parshall Flume, the Palmer-Bowlus flume, the Thelmar weir, and the inline weir. The purpose behind using a primary device is to be able to measure the flow rate from a single measurement—depth (also known as the level within the pipe as measured by the area-velocity meter). Depth is converted to flow from a rating curve established for the specific device and conditions. Depth is a fairly robust measurement, and very easy to confirm with alternative measurements such as a tape measure. The goal of each of these devices is to operate as a primary device over as wide a range as possible. In order to achieve that goal, the primary devices obstruct the flow of wastewater within the sewer systems to create a slow moving pool of water. measurable depth. In general the greater the obstruction to flow, the wider the range to which the rating curve will apply.
However, the current techniques that utilize these primary devices, as well as other methods that currently measure wastewater flow, have drawbacks. One drawback is that the devices used typically are not surface insertable, adding to the cost, labor, and limitations involved. Another drawback is that the current area-velocity meters have difficulty measuring very low flow, which places a size limitation on the precipitation events available for the corresponding methods.
A further drawback relates to the obstructions that current primary devices create in order to widen the range of measurable depth. This not only poses the risk of clogging up the sewage system, but also results in increased costs due to the inefficiencies of having to monitor these devices. For example, extremely obstructive devices, such as the Thelmar weir, are typically only used for a few hours and then removed to reduce the risk of obstruction. Devices less subject to clogging, like a Palmer-Bowlus flume, create deep flow upstream which results in a very slow flow, allowing solids to deposit upstream and become anaerobic. Some methods even require extended on-sight evaluations by consultants, which further increases costs.
Thus, there is a need for an accurate, efficient, and cost-effective method and device for measuring wastewater flow in sanitary sewer systems in order to pinpoint I/I sources.